The effects of β-NAD (A, B), adenosine (ADO, C, D) and ATP (E, F) in the absence (closed squares) and presence of the P1 receptor antagonist 8-p-sulfophenyltheophylline (8-SPT, 100 μM) in the canine mesenteric artery (left column) and vein (right column). Data represent mean±SEM from 2–6 experiments. The asterisks denote significant difference from controls in the absence of 8-SPT, P<0.05.